The invention relates to catadioptric objective and the use thereof in a microscope or a microlithographic projection exposure apparatus. The catadioptric objective includes spherical and aspherical lens elements and two concave mirrors which face each other. All components of the catadioptric objective, including also the object field and the image field, are arranged centered to a linear optical axis. This class of catadioptric objectives has a central aperture obscuration.
At wavelengths in the deep ultraviolet range, that is, wavelengths less than 250 nm, mirrors having a positive refractive power are used in combination with lenses of negative refractive power as suitable means for color correction.
A catadioptric microscope objective having two concave mirrors facing each other is disclosed in Russian patent publication 124,665. The 60xc3x97 magnification of the catadioptric microscope objective is achieved without intermediate imaging. Because of the low field size, only a few spherical lenses are needed for correction. A composite lens is used in addition to the mirrors for color correction. This correction means is, however, no longer available in the deep ultraviolet wavelength range.
Catadioptric objectives for microlithography having only one concave mirror are known from U.S. Pat. No. 5,691,802 or European patent publication 0,475,020. In these systems, the optical axis must be bent at least once. If reticle and wafer are to be mounted parallel to each other, then a two-fold beam deflection is required. This leads to significant complexity with respect to construction. If, in addition, a purely reflective beam splitter is used, such as disclosed in U.S. Pat. No. 5,691,802, then only off-axis object fields can be imaged. The lenses of the objective near to the field are non-symmetrically illuminated whereby asymmetrical thermal deformations and therefore imaging errors which are difficult to correct occur because of the absorption of the lenses.
A centered arrangement of the optical components on a linear optical axis having two concave mirrors facing each other as shown in FIGS. 1 and 2 does not have this disadvantage. In contrast, an aperture obscuration occurs because of the cutouts in the mirrors.
The effects of an aperture obscuration on the contrast transmission function is investigated in the article of S. T. Yang et al entitled xe2x80x9cEffect of Central Obscuration on Image Formation in Projection Lithographyxe2x80x9d (SPIE Volume 1264, Optical/Laser Microlithography III (1990), pages 477 to 485. For incoherent illumination, the contrast is reduced for low spatial frequencies in comparison to an unvignetted system. The acceptance of obscured objectives can therefore be significantly increased when the aperture obscuration is further reduced. In addition, a reduction of the contrast transmission function must not necessarily lead to a reduction of the resolution capacity because of the nonlinear response function of the photoresist. By suitably selecting the photoresist, the break in the contrast transfer function continues to lie above the exposure threshold of the photoresist.
It is an object of the invention to further reduce the aperture obscuration and the lens diameters in objectives of the kind described above. It is a further object of the invention to provide excellent imaging and color correction for the field sizes typical for microlithography and an increase of the image end aperture compared to the state of the art with the least possible use of material.
The catadioptric objective of the invention transmits a light beam along a light path and defines an optical axis. The catadioptric objective includes in sequence of the travel of the light beam: a first lens group having a negative refractive power and arranged centered on the optical axis; a first concave mirror having a central cutout and being arranged centered on the optical axis downstream of the first lens group; a second concave mirror having a central cutout and being arranged centered on the optical axis downstream of the first concave mirror; the first and second concave mirrors being disposed so as to face each other; a second lens group having a negative refractive power and being arranged centered on the optical axis downstream of the second concave mirror; the first lens group having a first plurality of lenses arranged upstream of the first concave mirror; the second lens group having a second plurality of lenses arranged downstream of the second concave mirror; and, one of the first and second plurality of lenses having at least one aspheric lens surface.
The catadioptric objective of the invention is normally combined as a partial objective with at least one dioptric (purely refractive) partial objective to form a reduction objective. The combination of a catadioptric component objective with at least one dioptric partial objective and the use in a microscope or in a microlithographic projection exposure apparatus is also described.
In the catadioptric objective, the light rays starting from the object plane first pass through a first lens group having a negative refraction power and then impinge on a first concave mirror which has a hole at its center. This concave mirror is mounted concavely to the object plane. The light is reflected back and impinges on the second concave mirror which likewise has a central hole. This second concave mirror is mounted concavely to the image plane. In this way, the two concave mirrors face each other. The light rays are reflected back from this second concave mirror and pass through a second lens group having a negative refractive power before they impinge on the image plane of this catadioptric partial objective.
The cutouts in the mirrors make a continuous ray trace possible but lead to a central obscuration in the illumination of the diaphragm plane. All rays which would impinge in the region of the mirror cutouts when reflected at the concave mirrors do not contribute to imaging and have to be vignetted via suitable measures. An obscuration of the aperture rays occurs. The rays which proceed from an object point are characterized as aperture rays and these rays lie within a bundle of rays delimited by the system diaphragm.
The first lens group, the two concave mirrors and the second lens group are arranged centered on a common optical axis defining a straight line. The aperture obscuration and the use of material for the lenses is further reduced because of the targeted use of aspheric surfaces.
By using with one or several partial objectives, the intermediate image, which is generated by this catadioptric objective, shows intense aberrations which are then compensated with the additional objectives in the total image. The catadioptric objective is to exhibit a chromatic overcorrection and/or overcorrection of the Petzval sum as a compensation for combination with dioptric partial objectives.
It is especially advantageous when the lens elements directly forward of the first concave mirror and/or the lens elements directly after the second concave mirror have an intense negative refractive power. The lens elements can be individual negative lenses or can be several lens elements which, however, have to exhibit an overall negative refractive power. It is advantageous when these lenses having negative refractive power or adjacent lenses have aspheric lens surfaces. These lenses with negative refractive power generate a chromatic overcorrection. The amount of the chromatic axial aberration for lenses having a refractive power "PHgr" and a marginal ray height hRD is proportional to hRD2xc2x7"PHgr" and the lenses having negative refractive power close to the mirror have a low marginal ray height because of the required low aperture obscuration. For this reason, the refractive power of the lenses has to be that much higher in order to achieve an adequate chromatic overcorrection.
It is advantageous when the lenses of the object end field lens group and/or the image end field lens group have at least one aspheric surface in order to influence the chief ray angle in the object plane and the image plane. All those lenses are counted to the field lens group within which the marginal ray of an object point of the optical axis runs between the optical axis and the chief ray of the outermost field point.
A further feature of the invention is that the first lens group can be subdivided into three subgroups. The center subgroup has a positive refractive power; whereas, the first and third subgroups have negative refractive powers. For the correction, it advantageous when a lens of the center subgroup has an aspherical surface.
To hold the lens diameter small, the lenses of the first and second lens group advantageously are passed through only in one direction. Lenses, which are run through a multiple number of times, should be in the light path between the concave mirrors and have correspondingly large diameters. However, this does not preclude that the first and/or second lens group extend partially into the space between the mirrors.
If the absolute value of the magnification ratio of this catadioptric objective lies approximately between 0.70 and 1.30, then an assembly symmetrical to the diaphragm plane can be achieved for the concave mirrors and their central cutouts. In this way, the diameters of the cutouts and therefore their contributions to the aperture obscuration are similarly large. If the magnification ratio does not lie in this region, then the first and second lens group have different focal lengths and it is significantly more difficult to maintain the diameters of the cutouts the same size. A further advantage of an absolute value of the magnification ratio between 0.7 and 1.3 and of a configuration symmetrical to the diaphragm plane is the fact that asymmetrical imaging errors such as coma can be well controlled in a low order.
The minimum aperture obscuration is achieved when the last lens of the first lens group and the first lens of the second lens group are each arranged in the region of a mirror cutout. It is therefore advantageous when individual lenses or at least lens parts are located in the geometric space between the first and second concave mirrors.
A concave surface which comes close to a hemisphere is advantageously provided in the first and/or second lens group. The ratio of half a lens diameter to the radius of the surface is greater than 0.70 for these surfaces. The concave surface of the first lens group then faces toward the object plane and the concave surface of the second lens group faces toward the image plane. The aperture obscuration can be held very low because of these very intense diverging surfaces close to the concave mirrors.
It is advantageous that the lens having the concave surface or an adjacent lens has an aspherical lens surface. The aperture obscuration can be further reduced with the aspherical surface in the proximity of a lens having a high negative refractive power.
The maximum deflection of the marginal ray within the first lens group and/or within the second lens group can be achieved with low aperture obscuration when a lens surface is provided in close proximity to the concave mirrors so that the angles of incidence of the marginal rays referred to the surface normal assume maximum values at the particular passthrough point. The angles of incidence are limited upwardly only by the necessity of an antireflecting coating, which is adapted for the particular working wavelength, and unwanted polarization effects for angles of incidence close to the Brewster angle. In this way, the sines of the angles of incidence for these lens surfaces result which, in any event, are greater than the object end numerical aperture by a factor of three.
It is advantageous when the lens has a surface, which has high angles of incidence, or an adjacent lens has an aspherical surface. In this way, aberrations can be compensated which are caused by the surface having high angles of incidence.
It is possible to reduce the aperture obscuration to values below 35% and even down to 20% with the aspheric surfaces and the lenses having high negative refractive power in the proximity of the concave mirrors. The aperture obscuration is defined as a percent ratio of the sine of the ray angle of a ray which proceeds from an object point on the optical axis and is targeted to the edge of the mirror hole and the sine of the ray angle of the marginal ray of the same object point. The ray angles are determined with respect to the optical axis.
If the concave mirrors are arranged forward and rearward of the diaphragm plane, then the holes of the mirrors are almost symmetrical to the diaphragm plane. With this measure, the field-dependent aperture obscuration can be minimized. With the diaphragm plane between the concave mirrors, the chief rays intersect the optical axis after the reflection at the first concave mirror and forward of the reflection at the second concave mirror.
The catadioptric objective is so configured that a ray trace, which is substantially symmetrical to the diaphragm plane, results between the first and second lens groups so that the ray height of the marginal ray, which proceeds from an object point on the optical axis, at the last surface of the first lens group differs from the ray height of the same marginal ray at the first surface of the second lens group by a maximum of 20%.
A large aperture expansion is required in order to hold the aperture obscuration as low as possible and to reduce the structural length of the objective. If one looks at a ray, which intersects the optical axis in the object plane, then the aperture expansion can be defined as the ratio of the sine of the angle i2 of this ray after the first lens group to a sine of the angle i1 of the same ray forward of the first lens group. The angles are then each determined with respect to the optical axis. With a negative refractive power of the first lens group, the aperture expansion   m  =      |                  sin        ⁡                  (                      i            2                    )                            sin        ⁡                  (                      i            1                    )                      |  
is adjusted to greater than 2.0.
With aspheric surfaces, it is possible to control the aperture expansion in dependence upon the angle of the ray impinging upon the first lens group. With very high values for the aperture expansion and lenses having high negative refractive power, a significantly higher aperture expansion results for purely spherical lens surfaces for the marginal rays than, for example, for a paraxial ray. The increase of the aperture expansion with increasing ray angle leads to larger mirror diameters and therefore to an increase of the objective dimensions. With aspherical lens surfaces forward and rearward of the concave mirrors, the aperture expansion can be influenced in dependence upon angle. The object is that the ratio mR/mP is less than 1.1 and preferably less than 1.05 for the aperture expansion mR for a marginal ray and for the aperture expansion mP for a paraxial ray. The aspherical surfaces forward and rearward of the concave mirrors are so configured that the effect, which is generated by the aspherical surface of the first lens group, is compensated to a large extent by the aspherical surface of the second lens group. The action of the aspherical surfaces cannot be viewed completely in isolation but only in combination with the neighboring system surfaces.
The lens diameters can be greatly reduced in comparison to the mirror diameters with the intense aperture expansion of the first lens group and the corresponding aperture reduction of the second lens group. Material which is transparent in the low ultraviolet range and has large diameters is very expensive and is only available to a limited extent. For this reason, it is advantageous when the maximum lens diameters amount to only 20% to 25% of the mirror diameters.
For coupling the catadioptric objective to the illuminating system, it is advantageous when the catadioptric objective has an almost homocentric entrance pupil. The deviation of the object end pupil function from a line fit through this pupil function can serve as an index. The object end pupil function is understood to mean the trace of the tangent values of the chief ray angles in the object plane over the intersect heights of the chief rays in the object plane. An objective having a homocentric entry pupil would exhibit a linear pupil function. The compensating line is determined from all ray-angle ray-height value pairs in the region from xe2x88x92Ymax to +Ymax wherein Ymax is the maximum possible object height of the circular-shaped object field. The deviation of the pupil function from the line fit should and can be maximally xc2x110 mrad, preferably maximally xc2x15 mrad.
It is possible that all lenses are of the same material because of the color correction with the aid of both concave mirrors and the mirror-near lenses having high negative refractive power.
With wavelengths xe2x89xa6250 nm, preferably the fluoride crystals CaF2, BaF2, SrF2, LiF, NaF, KF can, inter alia, be used in addition to special quartz glasses and mixed crystals.
In the intermediate spaces between the lenses, a gas is advantageously provided which exhibits only a slight absorption in the region of the working wavelength. The gas charge can be provided with synthetic air, N2 or rare gases in dependence upon working wavelength. Large light paths occur in the space between the concave mirrors and disturbances because of refractive index fluctuations and pressure fluctuations have a great influence. For this reason, the space between the concave mirrors is preferably filled with a gas whose refractive index exhibits a lower temperature dependency and pressure dependency than nitrogen. Helium is ideally suited.
It is purposeful to couple the described catadioptric objective with a refractive objective in a reduction objective. The catadioptric objective of the invention defines a first partial objective which images the object plane into an intermediate image plane. The intermediate image plane is imaged demagnified with a refractive second objective on the image plane. The magnification ratio of the reduction objective typically lies in the range from xe2x88x920.1 to xe2x88x920.5. The sequence of catadioptric and refractive component partial objectives can also be reversed. In the catadioptric partial objective, the chromatic axial aberration and the image field curvature are so overcorrected by the lenses, which are adjacent to the concave mirrors having intense negative refractive powers so that a corrected image results in the image plane of the reduction objective with reference to chromatic aberration and image field curvature. The lenses of the refractive partial objective correct the image errors spherical aberration, oblique spherical aberration and coma for an image field greater than 20 mm and an image end aperture greater than NA=0.7.
The intermediate image by the catadioptric partial objective is only inadequately corrected. Aberrations such as a large inner coma are present in the intermediate image because of the intense aperture expansion and the lenses having high negative refractive power. To correct these aberrations, which are introduced by the catadioptric partial objective, and to provide adequate correction of the large image field and the high numerical aperture, it is advantageous when the refractive partial objective has at least one aspherical surface.
Two aspherical lens surfaces are arranged symmetrically to the diaphragm plane and permit a correction of the spherical aberration as well as the correction of the field-dependent aperture aberration such as coma and oblique spherical aberration. The two aspherical lens surfaces are arranged in such a manner that a chief ray has at both surfaces a similarly large ray height with respect to magnitude and the difference in the ray heights amounts to maximally 30% and preferably less than 20%.
In these and other objectives, it is advantageous to provide two mutually adjacent aspherical surfaces to correct the spherical aberration and the sine condition. The two aspherical surfaces can define the two sides of a lens or they can be provided on two lenses and lie separated from each other by an air space. These double aspheres close to the diaphragm plane are especially effective so that the ray heights of the chief ray of the outermost field point at the adjacent aspherical surfaces are maximally 15%, preferably maximally 10% of the diameter of the diaphragm.
If the refractive partial objective is disposed forward of the image plane of the entire objective, then it is advantageous to provide an aspheric surface in the field lens group next to the intermediate image plane in order to influence the chief ray angles in such a manner that a telecentric trace of the chief rays results at the image end. The chief ray angles with reference to the image plane should lie within the entire image field in the range of xc2x15 mrad. The object end field lens group of the refractive partial objective includes all the lenses within which the marginal ray of an object point of the optical axis runs between the optical axis and the chief ray of the outermost field point.
Because of the chromatic overcorrection of the catadioptric partial objective, it is possible to utilize only lenses made of one material in the refractive partial objective even for a bandwidth of the light source of several pikometers (up to 10 pm).
The reduction objective is formed from the catadioptric partial objective and the refractive partial objective and can be used also in a microscope because of the high numerical aperture of NA greater than 0.70. In the reduction objective, the object plane and the image plane are to be exchanged, that is, the objective is to be operated in the opposite direction. A further increase of the aperture can be achieved with a reduction of the field size.
The objective can be used for the inspection of wafers with small fields, very high apertures and wavelengths less than 250 mm. This can take place in the context of a narrow wavelength band or a broad wavelength band.
Usually, this class of catadioptric reduction objectives is used in microlithography. The objective is then a component of the microlithographic projection exposure system. The following lasers can be used as light sources in the DUV/VUV wavelength range: ArF laser for 193 nm, F2 laser for 157 nm, Ar2 laser for 126 nm and NeF laser for 109 nm. An illuminating system ensures the homogeneous illumination of the structure mask. The field lens group of the illuminating system functions to adapt the exit pupil of the illuminating system to the entrance pupil of the projection objective. The illuminating system additionally includes means for controlling the partial coherence and for field masking the structural mask.
Microstructural components having structural sizes even below 0.1 xcexcm can be produced with a microlithographic projection exposure system of the above kind.